Locomotion in humans must be flexible enough to accommodate changing environment demands and task constraints. Achieving this requires modification of intra- and inter-limb coordination without loss of stability. Reactive changes rapidly occur using peripheral feedback (e.g., increasing your step height to clear a curb after you catch your toe on it). Slower adaptive changes depend on practice and occur over minutes to hours (e.g., changing your walking pattern to adjust to new shoes). They result in new calibrations of feedforward motor commands, which cause after-effects that persist when the demands are removed.
Normally, both types of locomotor adjustments can be made with ease. For example, when people walk on a split-belt treadmill that moves each leg at a different speed, there is an immediate reaction such that the slower leg spends more time in stance and the faster leg spends less time in stance. This reaction persists during split-belt walking, and then immediately reverses when the belts are returned to normal treadmill conditions (i.e., the belts tied at the same speed). In contrast, step lengths also are initially asymmetric, but an adaptive response occurs during split-belt walking that acts to re-establish symmetry via feedforward changes in phasing between legs. This adaptation induces an after-effect, causing walking asymmetry when returned to normal treadmill conditions.
The leg speed asymmetry is a part of natural regulation of the change in the heading direction. The differences in speeds have previously been viewed as a complication to the explanation of turning strategies; however, the velocity-dependent turning has now been explained by the dynamics of central pattern generation within the spinal neural circuitry. During the locomotion on the split-belt treadmill, the quality of robust turning control relies on the congruent experience of changes in the peripheral flow of visual information, the appropriate detection of limb speeds, and the success of decoupling the twisting action at the hip from stepping modifications.
With the advent of modern-day exercise machines, self-paced treadmills were created that enable the continuous adjustment of speed based on the user's own performance and adjustments. The original goal of self-paced systems was to keep the user in the center of the treadmill and automate safety measures. While these systems were the first to utilize feedback algorithms that reported on the subject's position relative to the treadmill, they almost always used external devices that facilitated motion/video capture, such as an ultrasonic range finder or a feedback-controlled locomotion interface. The sensor can be implemented with a force transducer in line with a tether connecting subject to the front of treadmill.